Hey Guys,

I have a 00 Si. The motor is stock aside from an AEM intake. Ive been having these problems with the motor stuttering. It happens when Im at mid rpm range, like 3k-5k (for the most part), either accelerating or cruising on the highway. Suddenly the engine will start decelerating, kinda like as if the car has run out of gas, or like one of the cylinders went out. When this happens, the engine is not making enough power to accelerate, even if I really give it throttle. It lasts either for a few seconds happening quickly on and off, or just the occasional stutter/hickup. One time it was particularly bad though. I started the car up in the morning, and the car was shaking like some cylinders werent working. I saw smoke behind the car, but i wasnt sure what color it was since the windows were fogged. The engine was running so badly, and i gave it gas and even at full throttle it was struggling to increase revs, and slowly made it to about 2500rpm in neutral before i decided to stop torturing it. I just left, as I was in a rush, and about a minute after starting it the engine returned to normal.

Ive tried a bunch of things already. The car has 89,000 miles on it, but the problem has been there for easily the last 30-35k miles. It comes and goes, so its hard to pinpoint a cause. I have changed the plugs many times, i have changed the distributor cap and rotor, the plug wires, and the fuel filter twice. At one point during an oil change, i asked mechanics to look into it themselves, as i was stumped, and they couldnt replicate the problem so they couldnt fix it, but they said that the plugs showed the engine had been running incredibly rich at some point, which i presume is when all this stuttering occurs.

Today I had some thoughts about the possibility that all these things were ok, and that maybe some sensor was going bad and so it was forcing the ecu to do silly things every now and then, but im just not sure. Its a strange problem, and what makes it stranger is the randomness with which it occurs. Its happened a few times to me on the highway with cruise control on - suddenly the car will just start slowing down and i can hear the cruise trying to maintain the speed. As far as gasoline brands, I have tried to stick to a "good" or rather expensive brand such as chevron or shell, so I doubt im running into 2 years of bad gasoline. Im really stuck otherwise though - the problem is so rare that it cant be replicated for anyone really, so Im wondering what you guys on here think it could be?

Thanks for reading my post!

 

Maybe a bottle of injector cleaner would help.

Since you have done full tuneups on it, I can't really recommend anything else. What octane gasoline are you using?

 

Im using 93 octane gas - always the premium gas at whichever station I go to. I tried checking the MAP and IAT wiring was connected properly, and then I disconnected the MAP sensor and started the car for a laugh. The CEL of course came on, and I noticed there was a slight stumble and gentle stuttering when I went to about 50-75% throttle from idle as it climbed through 2k rpm - it was sorta reminiscent of the way the engine would react when the stuttering was happening, except it was much less noticeable and let the engine keep revving. Im thinking that even if it is a MAP sensor going bad, that may not be the cause? It seems like the ecu can bandaid the loss of MAP sensor reading, whereas it could not bandaid whatever was happening to the car, especially on that morning. Im thinkin the MAP sensor might not be the issue. The other crazy thing is that I havent had any CEL's through any of it. I was looking through the Helms manual for the car and on pg 11-84 it lists a symptom chart for troubleshooting symptoms which will not cause the CEL to come on. Among the listed symptoms are "Misfire or Rough Running." I looked into the sections it mentioned, and it refers to the PGM-FI System section on Random Misfire. In the symptoms and causes chart, it says that for a misfire which occurs at a non-specific time (could be accel, low rpm, high rpm, etc...) check the following in the following order:

1) Fuel pressure
2) Distributor and Ignition Wires (already replaced)
3) MAP Sensor
4) ICM (Ignitor)
5) Valve Clearance and Crankshaft Position Sensor

Im inclined to rule out #1 since if there was a FP problem, it would be more persistent, and easier to replicate. #2 is ruled out since Ive already replaced them. I dont know if the MAP sensor could even cause this, since it seems like the worst it could do is run a bit rough, and even then it should thrown a CEL. #4 I guess is a possibility, and since they dont come too cheap, I havent replaced it yet. As far as Valve Clearance, I havent had the valves adjusted ever, but I will probably get that done along with the timing belt in a few weeks time.

Basically this doesnt seem to help alot - according to the book I should check out the ICM, but does anyone else have any ideas, thoughts, or experiences with anything like this?

Thanks again guys!

 

If your MAP sensor was bad, it would show a CEL, so that isn't your problem. The engine will have a hard time without the MAP sensor working because it relies on that as a signal to pull fuel or add fuel and ignition timing.

Another thing you may want to check as it just popped into my head is the ignition timing. Hook up a timing light and make sure that the distributor timing is to stock specs. I know that I was having a stumble under boost and the ignition timing was retarded beyond stock a few degrees, so it may be another place to look.